Stent and Method and Device for Fabricating the Stent

ABSTRACT

Stent, as well as a method and device for fabricating the stent, wherein the stent has a tubular lattice structure comprising individual struts and at least one strut of which at least one longitudinal section runs with at least one directional component in the radial circumferential direction of the stent, wherein the surface of the longitudinal section facing the outside of the stent is curved only about the longitudinal axis of the stent. According to the invention, the surface of longitudinal section of the strut, which surface faces the inside of the stent, has such a curvature that the strut cross section is fluidically optimized.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of priority to German patent applicationnumber DE 10 2008 038 367.8, filed on Aug. 19, 2008; the contents ofwhich are herein incorporated by reference in their entirety.

FIELD OF THE INVENTION

This invention relates to a stent that comprises struts, a method offabricating the stent, as well as a device for implementing the method.

BACKGROUND OF THE INVENTION

Stents are typically endovascular prostheses that are used for thetherapy applied to stenoses. They essentially comprise a supportstructure by which the wall of a vessel, such as, for example, anartery, is braced so as to ensure sufficient flow through the vessel. Inaddition, an aneurysm can be bridged by the stent. During implantation,the stent is inserted in a compressed state into the vessel and thenexpanded at the site to be treated. Expansion of the stent is typicallyeffected by means of a balloon catheter that has been previouslyinserted into the interior of the stent and also functions to positionthe stent within the vessel. As a result of the expansion of the stent,the walls of the support structure are pressed against the vascularwall, thereby effecting an adequate sectional area of flow for thevessel. In order to ensure that the sectional area of flow is notreduced too much by the stent itself, stents generally have very narrowwall thicknesses in the region of the support structure. These narrowwall thicknesses must, however, ensure that the expanded shape of thestent is preserved despite a pressure applied by the vessel and actingradially on the stent. In addition to radial strength, the requirementalso to be met by the stent is to have sufficient flexural stiffness toenable the stent to adapt as well as possible to the partially bentshape and the movements of the vascular section in which it isimplanted.

Typically, the support structure of the stent here is essentiallylattice-shaped, although this lattice can have a wide variety ofdesigns. The lattice structure is typically generated by a laser cuttingprocess targeted at a lateral cylindrical surface of a tube.

The struts forming the lattice structure thus have an essentiallyrectangular cross section, as shown in FIG. 2 which illustrates theprior art. The disadvantageous aspect of the embodiment of the strut 200shown in FIG. 2 is that the blood flow 211 passing over strut 200 tendsto form turbulences 213, with the result that a turbulent flow iscreated in the region of strut 200 instead of the desired laminar flow.Turbulences 213 result in unwanted deposits along with the possibleconsequential effects of neointimal hyperplasia and arthrosclerosis 212.These deposits can develop further into a symptomatic restenosis. Thiseffect is found particularly in the case of proximally-located strutsand decreases in the distal direction.

Various measures are known involving treating the struts after thelaser-cutting procedure. The struts are treated, for example, byelectropolishing. The rounding of edges produced thereby is so small,however, that it does not produce any significant change in the flowfrom turbulent into laminar. In addition, electropolishing is used totreat the entire support structure, with the result that it is notpossible to perform a selective treatment of the proximal end of thestent. Furthermore, the slight removal of material here essentially iseffected symmetrically over the strut, that is, on the mural edges, theedges facing the vascular wall, and on the luminal edges facing thevascular lumen—all of which does not counteract the formation ofturbulences.

An ellipsoid cross-sectional surface for the strut is known from EP 0824 903 A1 which differs from the cross section of a strut as describedand illustrated in FIG. 2. A cross-sectional shape is shown in FIG. 2 ofthis publication in which both the luminal as well as the mural surfacesare curved around the longitudinal axis of the strut. It is thecurvature of the mural surface in particular that is disadvantageous inthat a strut curved in this way penetrates more deeply into the vascularwall during dilatation, thereby damaging this wall to a greater extent.The result is an increase in neointimal proliferation which in turnpromotes the formation of deposits. The luminal surface of the strut iscurved symmetrically. It is well known that symmetrical curvatures havean adverse effect on fluid flows in so far as they are unable to producea laminar flow. This means that turbulences of the blood flow passingover the luminal surface occur even in this ellipsoid embodiment shownin EP 0 824 903 A1, which turbulences in turn also result in theformation of deposits. Another disadvantage of the strut cross sectionsreferenced in this document is the reduction in its axial sectionmodulus which in particular has a disadvantageous effect on the strengthof the stent in response to increased bending stress. In addition, theradial strength of the stent is lowered by the decreased wall thicknessof the support structure.

Particularly in the case of novel stents composed of magnesium alloys,relatively large wall thicknesses are required for reasons of strength,with the result that the cross-sectional shapes of the struts disclosedin EP 0 824 903 A1 are not usable for such stents without undulyreducing the radial strength of the stent. In order to fabricate stentsfrom magnesium alloys that have the requisite radial strength and usingthese types of web cross sections as known in the referenced prior art,the struts would have to have significantly larger dimensions in crosssection. This is disadvantageous, however, because the sectional area offlow of the blood vessel is reduced thereby, which effect results inturbulences and increased formation of deposits.

US 2008/0082162 A1 discloses various strut cross sections, althoughthese have a coating on their mural surface. Curvatures of luminal strutsurfaces are of symmetrical form, as a result of which once again thereferenced unwanted deposits appear on the strut when inserted in theblood stream.

The fundamental problem to be solved by this invention is to provide astent, as well as a method of fabricating the stent, wherein theobjective is to design the stent such that vascular constriction isminimized while at the same time sufficient flexural and radialstiffness of the stent is provided.

SUMMARY OF THE INVENTION

According to the invention, a stent is provided that has a tubular,individual-struts-comprising lattice structure, wherein the stent has atleast one strut, of which at least one longitudinal section runs with atleast one directional component in the radial circumferential directionof the stent, and wherein the radially outward-facing (mural) surface ofthe longitudinal section of the strut is curved only around onelongitudinal axis of the stent. According to the invention, the surfaceof the strut's longitudinal section (luminal) facing towards the insideof the stent has a curvature such that the cross section of the strut isfluidically optimized. The invention thus relates to the design of astrut, or also only of a longitudinal section of a strut, that runs withat least one directional component in a radial circumferential directionof the stent. This means that the strut, or its longitudinal section,runs parallel to a tangent applied to the circumference of the stent, orhas one component that is oriented in this direction. What is excludedaccording to the invention, however, are the longitudinal strut sectionsthat run only parallel to the longitudinal axis of the stent. Thisfundamentally distinguishes the stent according to the invention fromthe embodiments shown in EP 0 824 903 A1. These types of strutsaccording to the invention can be easily cut from a tube by laser. Whatis understood by the strut cross section is that cross section which isproduced by a section in the longitudinal axis of the stent through thelongitudinal section of the strut with one directional component in theradial circumferential direction, in other words, performed in thedirection of the longitudinal axis of the stent. According to theinvention, the strut in cross section has only on the luminal surface inits longitudinal direction a curvature which is designed to preventturbulences of the blood flow passing through the stent, and thusprevent deposits. On the mural side, the strut has no curvature in thecross section running in the longitudinal direction of the stent. Thisdesign is particularly advantageous since the mural contact surface ofthe strut on the vascular wall is not thereby reduced, and thus thepressure of the strut on the vascular wall is limited, thereby reducingthe risk of damage to the vessel.

In order to effect the optimal prevention of turbulences, the stentaccording to the invention should be designed such that the luminalsurface of the strut has an optimally curved surface. The optimalcurvature is found by the person skilled in the art based on tests orconventional flow simulation models and can be, among other factors, afunction of the sectional area of flow of the vessel and of the flowvelocity.

In a preferred embodiment of the stent according to the invention,provision is made whereby the stent's inward-facing curvature of thestrut's surface is designed to be asymmetrically convex. This means thatas viewed in cross section the curvature of the luminal surface can haveat every point a different distance from the mural surface of the strutsection, where the convex curvature is of asymmetric shape. In otherwords, the curvature of the luminal surface, as viewed in cross section,has different convex regions that determine the asymmetry. The asymmetryin the convex shape produces a streamlined design of the strut crosssection, as a result of which the blood flow passing through the stentflows laminarly in the region of the strut and deposits on the strut areprevented. It is specifically this laminar flow motion of the bloodeffected by the cross section that has the additional very importanteffect of promoting the endothelialization of the stent, which is the“in-growth” with the endothelial cells. This too results in theprevention of deposits.

Provision can be made here whereby the stent according to the inventioncomprises biode-gradable material in the region of the strut. In anotherembodiment, provision can be made whereby the strut is composed entirelyof biodegradable material. Those materials are defined as biodegradablewithin the meaning of the invention in which a decomposition takes placewithin a physiological environment, this decomposition ultimatelyresulting in a condition whereby the entire implant, or the part of theimplant composed of this material, looses its mechanical integrity.

Preferably, this biodegradable material is a biodegradable metal,preferably, a biodegradable alloy selected form the group magnesium,iron, and tungsten; in particular, the biodegradable material is amagnesium alloy. What is understood by alloy here is a metallicmicrostructure, the principal components of which are magnesium, iron,or tungsten. The main component is the alloy component in which theweight percentage of the alloy is the highest. A percentage for the maincomponent is preferably 50 wt. %, in particular. more than 70 wt. %.

If the material is a magnesium alloy, this preferably contains yttriumand other rare-earth metals since such an alloy excels in terms of itsphysiochemical properties and high bio-compatibility, also in particularin terms of its products of decomposition.

What is especially preferred for use is a magnesium alloy with thecomposition of rare-earth metals being 5.2-9.9 wt. %, of which yttriumis 0.0-5.5 wt. % and the remainder is <1 wt. %, where magnesium makes upthe missing fraction of the alloy up to 100 wt. %. Both experimentallyand in initial clinical trials, this magnesium alloy has confirmed itsspecial applicability, i.e., demonstrates high biocompatibility,advantageous working properties, good mechanical characteristics, and adegradation behavior that is suitable for the intended applications. Thecollective term “rare-earth metals” primarily denotes scandium (21),yttrium (39), lanthanum (57), and the following elements followinglanthanum (57), specifically, cerium (58), praseodymium (59), neodymium(60), promethium (61), samarium (62), europium (63), gadolinium (64),terbium (65), dysprosium (66), holmium (67), erbium (68), thulium (69),ytterbium (70) und lutetium (71). The alloys of the elements magnesium,iron, and tungsten must thus be selected in terms of their compositionso as to be biodegradable.

Alternatively, what can be employed in place of a material based onmetal is a biodegradable polymer. Preferred polymers for the polymermatrix of the implant according to the invention are selected from thegroup polydioxanone, polyglycolide polycaprolactone, polylactide(poly-l-lactide, poly-d,l-lactide, and copolymers and blends such aspoly(l-lactide-co-glycolide), poly(d,l-lactide-co-glycolide),poly(l-lactide-co-d,l-lactide), poly(l-lactide-co-trimethylenecarbonate, triblock copolymers), polysaccharides (chitosan, levan,hyaluronic acid, heparin, dextran, cellulose, etc.),polyhydroxyvalerate, ethylvinylacetate, polyethylene oxide,polyphosphorylcholin, fibrin, albumin.

Alternatively, provision can be made whereby the strut is composed of apermanent material that does not decompose. The base body of thepermanent stent is preferably composed of a metal material consisting ofone or more metals from the group iron, nickel, tungsten, zirconium,niobium, tantalum, zinc, or silicon, and optionally, a second componentconsisting of one or more metals from the group lithium, sodium,potassium, calcium, manganese iron, or tungsten, preferably consistingof a zinc-potassium alloy. In another exemplary embodiment, the basebody is composed of a shape-memory material consisting of one or morematerials from the group composed of nickel-titanium alloys andcopper-zinc-aluminum alloys, preferably, however, of Nitinol. In anotherpreferred embodiment, the base body of the stent is composed ofstainless steel, preferably consisting of a Cr—Ni—Fe steel—herepreferably the alloy 316L—or a Co—Cr steel. In addition, the base bodyof the stent can be composed at least in part of a plastic and/or aceramic.

In a first alternative embodiment of the invention, provision is madewhereby all of the strut sections of the stent that run with onedirectional component perpendicular to the longitudinal direction of thestent are designed in accordance with the present invention. As aresult, the embodiment of the strut according to the invention is notrestricted to specific sections of the stent; instead, all struts ofthose stents in general can be designed according to the invention whichhas a directional component perpendicular to the longitudinal directionof the stent. In a second alternative embodiment, the stent has strutsthat are designed according to the invention only at its end regions,where the end regions, starting from one stent end each, each extendover 20 to 30 percent of the length of the stent. This means that thestruts are designed in a fluidically optimized fashion only at the endregions of the stent. The center region of the stent—aside from theconventional electropolishing, which continues to be employed, used tosmooth the strut surfaces and to preclude sharp strut edges—thus doesnot undergo any surface treatment of the strut that significantlymodifies the strut cross section. This may be sufficient for simplestents since experience shows that the deposits occur only at the endregions of the sent (usually at the proximal end of the stent) and notat the center of the stent. This embodiment has the advantage that thewall thickness of the stent, that is, the material thickness of thestrut in the center region of the stent, is of thicker design than atleast at the proximal stent end. It is also possible to implement thestent such that a surface treatment of varying degree is performed overthe length of the stent, thereby making the cross-sectional area of thestrut larger as the strut 30 becomes more centrally located in thestent. This means that the removal of material so as to effect thefluidic optimization of the strut cross section becomes greater thefurther away a strut is located at the end of the stent. This in turnhas the advantage that the wall thickness is made thickest specificallyin that region of the stent in which the greatest bending stress isfound when a bending moment is applied to the stent end. As a result,the stent ends here remain flexible and can thus more easily adapt tomovements of tissue. Another advantage is that the transition betweenregion in which the vascular wall is supported by a stent of this designand the vascular wall immediately adjacent to the stent ends is “soft.”This means that the elasticity of the vessel increases or decreasesnonincrementally. In other words, an abrupt drop in elasticity betweenunsupported tissue wall and vascular wall is prevented. This minimizesmechanical irritation at the stent ends and in turn lowers the risk offocal stenoses at the stent ends.

In a preferred embodiment of the invention, provision is made wherebythe stent according to the invention is constructedmirror-symmetrically. This means that, for example, convex asymmetriesare arranged and shaped along the surface curvatures of the luminalstrut surfaces such that they have a different, in each case opposite,orientation on each half of the stent. The mirror plane here runsperpendicular to the stents longitudinal axis precisely at thelongitudinal center of the stent. Although based on this design one endof the stent is not in fact of optimal fluidic shape due to itsasymmetry and position within the vessel when inserted in the bloodstream, this disadvantage is nevertheless acceptable since this stentregion constitutes the end for the stent at which only minimal depositsoccur due to very small turbulences. The proximal end of the stent,however, is optimized in any case, with the result that deposits areprevented according to the invention at this end. This design has theadvantage that implantation of the stent is orientation-independentsince each of the ends of the stent is fluidically optimized and canthus form the proximal end of the stent independently of the orientationof the stent when in use.

In an alternative design, provision can be made whereby the asymmetriesof the curvatures on the struts are oriented unilaterally over theentire length of the stent. As a result, mirror symmetry of the stent nolonger exists, while on the other hand the advantage is created wherebyboth ends of the stent are of optimal fluidic design and thus anypossible minimally occurring deposits at the distal end can beprevented. However, care must be taken during implantation with thisdesign so as to ensure that the proximal end of the stent is theoptimized end in terms of the direction of flow.

Strut cross sections designed according to the invention can have athickness of between 60 to 185 μm, preferably, of between 60 and 90 μm.That means that the wall thickness of the supporting structure of thestent according to the invention also lies within these indicatedmicrometer ranges.

Another aspect of the present invention is the use of the stentaccording to the invention as an implant in a vessel to prevent vascularconstruction. The stent according to the invention thus relates to anembodiment that is fabricated such that it already has thefluidically-optimized curved surface on the luminal side of the strutduring implantation. This distinguishes the stent according to theinvention from embodiments in which a strut cross section that is atleast partially adapted to the blood flow does not result until afterrelatively long implantation of the stent due to material erosion of thestrut. By using the stent according to the invention, it is thuspossible to prevent deposits immediately after implantation.

A method is also provided according to the invention for producing thestent according to the invention wherein the lattice structure of thestent is produced such that at least one longitudinal section of thestrut running with at least one directional component in the radialcircumferential direction of the stent has an essentially angular crosssection, wherein a rounding is effected of the luminal edges, and edgesof the longitudinal section of the strut running with at least onedirectional component in the radial circumferential direction of thestent, said round being effected by at least one particle beam directedat the edges.

The lattice structure is generated in the conventional fashion by lasercutting of the tube. The particles of the particle beam here cancomprise fine-grained sand or spherical pellets composed of a solidmaterial. Due to scattering of the particle beam, the beam also strikesedges that are not located directly within the projection range of anozzle from which the particle beam emerges. The slight tumbling motionor gyrating motion of the particle beam ensures that the particlesstrike to a sufficient degree all strut edges located in front of thenozzle within the range of the beam. This thus enables a rounding to beproduced of the luminal edges of the strut such that these edges havethe fluidically optimized cross section according to the invention.

Provision is made advantageously here whereby the two luminal edges ofthe longitudinal section of the strut are each worked by one particlebeam, where the directions of the two particle beams are opposed to eachother.

The method is advantageously designed if the particle beam from onenozzle inserted into the stent is oriented towards the edge of the strutpointing towards the nozzle, and the nozzle withdrawn from the stent soas to effect sequential irradiation of multiple struts disposed side byside in the longitudinal direction of the stent. A scattering of theparticle beam is effected here such that all of the strut edges lyingwithin one section perpendicular to the longitudinal direction of thestent are irradiated simultaneously.

The irradiation of the edges here is effected bilaterally, where thenozzle is drawn at least once from the stent towards the first stent endand at least once from the stent towards the second stent end. Thenozzle here does not have to be inserted completely into the stent;instead, what is sufficient for treating the stent end regions is toinsert the nozzle only into this end region and to withdraw it from thestent as the particle beam is applied. Alternatively, the nozzle isinserted completely into the stent and drawn from the end opposite theinsertion end only through this opposing end region while applying theparticle beam, thereby irradiating this end region from the other side.

Both variants of the method should be employed to effect rounding of thetwo luminal edges of the longitudinal section of the strut. It ispossible here to perform both variants with different particledensities, different volumetric flow rates, different speeds forwithdrawal of the nozzle, and/or a different number of irradiationoperations so as to create the asymmetrically convex curvature of theluminal surfaces of the strut.

In order to effect the final treatment of the stent, provision is madewhereby the stent is treated by electropolishing after particle-beamirradiation. The electropolishing here functions to effect the roundingof the edges such that aside from luminal the edges the mural edges ofthe strut are also somewhat rounded so as to preclude any cutting intothe vascular wall. Despite the rounding of the mural edges, the muralsurface of the strut is nevertheless not modified comprehensively insuch a way that an overall curvature of the surface would result.Electropolishing also functions to effect the final surface smoothing ofthe entire support structure of the stent.

Provision is advantageously made whereby the speed of withdrawing thenozzle is varied. This means that, for example, when the nozzle iswithdrawn from one end region of the stent it is first withdrawn morequickly and then more slowly, thereby effecting a removal of lessmaterial at the side of the end region facing the center of the stentthan at the stent end.

According to the invention, a device is also provided to implement themethod according to the invention, wherein this device comprises areservoir to supply beam particles, an appliance to generate highpressure, a conduit to transport the particle beam, and a nozzleconnected to the transport conduit, wherein at least the nozzle has suchgeometrical dimensions so as to make it insertable into a stent. Thereservoir can, for example, be a tank inside of which there is highpressure. A conventional pump can be used to generate the high pressure.The transport conduit can be a tube or a pipe to which a nozzle isconnected. The transport conduit here can be designed such that thenozzle is created by an open end of the transport conduit as long asthis end is capable of being inserted into a stent and generating aparticle beam. In this case, the nozzle is an integral component of thetransport conduit.

The stent according to the invention can thus be rounded at its luminaledges by means of the method according to the invention and associateddevice such that the blood flow passes laminarly along the strutsdesigned according to the invention. The mural edges here are notrounded beyond what is achievable by standard electropolishing so as tokeep the mural support surface as large as possible and thereby minimizethe pressure on the vascular wall in order to prevent injury. Theasymmetrical cross-sectional shape of the strut is fluidicallyoptimized, wherein this can be designed according to fluidic simulationmodels. For example, an asymmetrical rounding of the luminal edges canbe effected at the stent end regions, wherein in particular the proximalend of the stent is fluidically optimized. As a result, the formation ofdeposits along the strut is prevented and the risk of unwantedneointimal hyperlasias and arteriosclerotic phenomena is reduced. Due tothe fact that the removal of material occurs preferentially at the stentend regions, the wall thickness of the support structure is reduced onlyin these stent end regions. This means that the center region of thestent has a wall thickness of the support structure that matches thewall thickness of the tube from which the support structure has been cutout. The result is a reduced flexural stiffness at the stent ends and aby comparison increased flexural stiffness at the stent center. Theseproperties are advantageous in particular in the event the stent when inuse must follow motions of the vessel in which it has been inserted.This means that the stent designed according to the invention can moreeasily participate in the motion of the vessel, thereby avoiding breaksor gaps between the stent end and the vascular wall resting thereon,with the result that the risk of deposits' forming in this region isreduced.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described below based on the attached drawings. In thedrawings:

FIG. 1 illustrates a section of a possible support structure of a stentaccording to the invention.

FIG. 2 illustrates a cross section of a conventional strut in userunning perpendicular to the longitudinal direction of the stent.

FIG. 3 illustrates the cross section of a strut designed according tothe invention and running perpendicular to the longitudinal direction ofthe stent;

FIG. 4 illustrates a schematically drawn stent comprising a nozzleinserted therein and particle beam.

FIG. 5 is a schematic view showing individual treatment stages of thestrut.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1 and 2 illustrate struts 200 such as those conventionally used inthe support structure or lattice structure 110 of a stent 100. Theinvention here is not restricted to the design of the lattice structure110 illustrated in FIG. 1; instead, the invention can comprise all stentstructures that have struts and in which the longitudinal sections haveat least one directional component that runs in the radialcircumferential direction of the stent. This refers to thoselongitudinal sections 220 that, as shown in FIG. 1, do not run parallelto the longitudinal axis 150 of stent 100. This thus refers tolongitudinal sections 220 that obviously run in a radial circumferentialdirection but also to the longitudinal sections that run obliquelyrelative to the longitudinal axis since these longitudinal sections havealso a directional component that runs perpendicular to the longitudinalaxis.

It is thus evident in FIG. 2 that the surfaces of the strut facing theoutside of the stent, that is, the mural surfaces that that contact thevascular wall 210, do not have any curvature in the cross section shown.This means that these surfaces are curved only in one direction,specifically, about longitudinal axis 150. In the stent according to theinvention, curvature of the mural strut surfaces is not affected aboutaxes that are perpendicular to longitudinal axis 150.

The invention is made clear in particular by a comparison of thecross-sectional shape of the strut in FIG. 3 and the conventionalcross-sectional shape in FIG. 2. Because of the luminal edges 223 onconventional strut 200, as shown in FIG. 2, a turbulence 213 of theblood stream 211 is created that results in the formation of deposits212 between vascular wall 210 and strut 200. These deposits can havearteriosclerotic effects and must be reduced or prevented.

The cross section of strut 200 is optimized by the design of strut 200according to the invention, as shown in FIG. 3, whereby a laminar bloodflow 211 is formed. Deposits are thereby prevented.

It is evident in FIG. 3 that the cross section of strut 200 is designedto be asymmetrically convex. Formation of the laminar flow is promotedin particular by the asymmetry.

FIG. 4 illustrates the way in which the fluidically optimized roundingof the struts can be produced. A schematically illustrated stent 100 isshown that is formed from a plurality of struts 200 similar or identicalto the design shown in FIG. 1. For reasons of clarity, only one stent200 is shown in cross section on each side of the stent, which crosssection is also shown in enlarged views in FIG. 5.

To implement the method according to the invention, a transport conduit300 is inserted into stent 100 at the end of which a nozzle 310 isdisposed. Transport conduit 300 conveys particles 400 to nozzle 310 fromwhich particles 400 exit as a particle beam 410. Particle beam 410exhibits a certain scattering, with the result that particles 400exiting nozzle 310 laterally strike struts 200 and also the luminalsurface of longitudinal section 221 of strut 200. What is achieved byimpinging particles 400 is that luminal edges 223 are rounded. Nozzle310 is withdrawn from stent 100 in the direction of second stent end130. Alternatively, the nozzle can also be pushed through stent 100towards the first stent end 120, although here care must be taken thatno obstruction in the displacement of nozzle 310 is effected by residualparticles 400 remaining in stent 100 due to the irradiation procedure.

As a result of repeated or sustained irradiation of strut 200, as isillustrated in FIG. 5 in the separate diagrams showing an enlargement ofregion X from FIG. 4, it is evident that a luminal edge 223 of strut 200is being rounded. What results is a curvature 222, the radius of whichbecomes increasingly greater the longer, or more frequently, the beam isdirected at strut 200.

Separate diagrams Y1 through Y3 in FIG. 5 shows strut 200 in an enlargedview of the region in FIG. 4 identified by Y. It is evident that instrut 200 shown here rounding has already taken place on the two luminaledges 223, with the result that a curvature 222 has already formed onthe luminal surface of longitudinal section 221, which curvature, as isespecially evident in FIG. 5, is of asymmetrically convex shape. Inorder to achieve this cross-sectional shape of the strut shown by Y3 inFIG. 5, nozzle 310 is first withdrawn, as shown in FIG. 4, from firststent end 120 towards second stent end 130, then removed from stent 100.This operation can be repeated. What results thereby is thecross-sectional shapes indicated in diagrams X1 through X3.

To effect the rounding of the still-present luminal edge 223, nozzle 310is drawn in a manner analogous to that described for second stent end130 towards first stent end 120, then withdrawn from the stent. Whatresults is, as shown in the cross-sectional shape illustrated for Y3 inFIG. 5, an additional removal of material in response to prolonged orrepeated irradiation, this removal resulting in the asymmetricallyconvex shape of the cross section.

The invention is not, however, restricted to this procedure; insteadprovision can be made whereby nozzle 310 is drawn only from stent center140 respectively towards first stent end 120 and second stent end 130,thereby rounding corresponding luminal edges 223. Preferably, provisioncan be made whereby only those struts 200 are rounded which are disposedat the two end regions 122 and 132, with the result that struts 200located at stent center 140 are not rounded by particle beam 410.

In addition, provision can be made whereby stent 100 is ofmirror-symmetrical design such that its two halves are of symmetricaldesign along mirror-symmetrical axis 160. In this case, the struts 200shown in regions X and Y in FIG. 4 are fabricated such that their convexcurvatures have different and opposing orientations. This type of designhas the advantage that the stent according to the invention can beimplanted in the vessel in an orientation-independent manner since itsproximal end 134 is in any case of fluidically optimized design. Thesomewhat less advantageous design of distal end 124 in this case doesnot have any disruptive effect since no deposits, or only minimaldeposits, are to be expected at the distal end.

Alternatively, the stent can be designed such that all asymmetricallyconvex curvatures have the same orientation. This means that all strutcross sections can have, for example, the shape illustrated in Y3 ofFIG. 5. This design has the advantage of the fluidically optimized shapeof all struts 200, although care must still be taken during implantationof the stent that stent 100 is implanted in the vessel such that the endoptimized in terms of the direction of flow is proximal end 134 of stent100.

It is evident in the diagrams of FIG. 5 that the mural surface oflongitudinal section 225 is not rounded by the particle beam.

It will be apparent to those skilled in the art that numerousmodifications and variations of the described examples and embodimentsare possible in light of the above teaching. The disclosed examples andembodiments are presented for purposes of illustration only. Therefore,it is the intent to cover all such modifications and alternateembodiments as may come within the true scope of this invention.

LIST OF REFERENCE NOTATIONS

-   stent 100-   lattice structure 110-   first stent end 120-   first end region 122-   distal end 124-   second stent end 130-   second end region 132-   proximal end 134-   stent center 140-   longitudinal axis 150-   mirror-symmetrical axis 160-   strut 200-   vascular wall 210-   blood flow 211-   deposit 212-   turbulence 213-   longitudinal section 220-   surface of the luminal longitudinal section 221-   curvature 222-   luminal edge 223-   mural surface of the longitudinal section 225-   transport conduit 300-   nozzle 310-   particle 400-   particle beam 410

1. A stent, comprising a tube-like lattice structure comprisingindividual struts, wherein the stent has at least one strut from whichat least one longitudinal section runs at least one directionalcomponent in the circumferential direction of the stent, wherein thesurface of the longitudinal section facing the outside of the stent iscurved only about the longitudinal axis of the stent, and wherein thesurface of the strut longitudinal section, which surface faces theinside of the stent, has such curvature that the strut cross section isfluidically optimized.
 2. The stent according to claim 1, wherein thecurvature of the surface of the strut, which curvature is directedtowards the inside of the stent, is of asymmetrically convex shape. 3.The stent according to claim 1, wherein the strut comprises abiodegradable material.
 4. The stent according to claim 1, wherein allof the strut sections of the stent run with at least one directionalcomponent in the radial circumferential direction of the stent.
 5. Thestent according to claim 1, wherein the at least one strut comprises twostruts, each positioned only at end regions of the stent, wherein theend regions each extend over 20-30% of the length of the stent, startingfrom each stent end.
 6. The stent according to claim 5, wherein thestent is constructed in mirror-symmetrical fashion.
 7. A method offabricating a stent, wherein the stent comprising a tube-like latticestructure comprising individual struts, wherein the stent has at leastone strut from which at least one longitudinal section runs at least onedirectional component in the circumferential direction of the stent,wherein the surface of the longitudinal section facing the outside ofthe stent is curved only about the longitudinal axis of the stent,wherein the lattice structure of the stent is produced such that atleast one longitudinal section of a strut running with at least onedirectional component in the radial circumferential direction of thestent has an essentially angular cross section, characterized in that arounding of the luminal edges of the longitudinal section of the strut,and of said edges running with at least one directional componentperpendicular to the longitudinal direction of the stent, is effected byat least one particle beam directed at the edges.
 8. The method offabricating a stent according to claim 7, wherein the two luminal edgesof the longitudinal section of the stent are each treated by at leastone particle beam, wherein the directions of the two particle beamsoppose each other.
 9. The method of fabricating a stent according toclaim 8, wherein the particle beam from a nozzle inserted into the stentis targeted at the edges of the strut facing the nozzle, and the nozzleis withdrawn at a predetermined speed from the stent so as to effectsequential irradiation of multiple struts disposed side by side in thelongitudinal direction for the stent.
 10. The method of fabricating astent according to claim 8, wherein the irradiation of the edges iseffected bilaterally, and wherein the nozzle is drawn at least once fromthe stent towards the first stent end and at least once from the stenttowards the second stent end.
 11. The method of fabricating a stentaccording to claim 8, wherein after particle beam irradiation the stentis treated by means of electropolishing.
 12. The method of fabricating astent according to claim 9, wherein the withdrawal speed of the nozzleis varied.
 13. A device for implementing the method according to claim7, wherein the device comprises: a reservoir to supply particles of thebeam; an appliance to generate high pressure; a conduit to transport theparticle beam; and a nozzle connected to the transport conduit toproduce the particle beam as a jet, wherein at least the nozzle is ofsuch geometrical dimensions so as to make it insertable into a stent.